In conventional radical polymerization processes, the polymerization terminates when reactive intermediates are destroyed or rendered inactive; radical generation is essentially irreversible. It is difficult to control the molecular weight and the polydispersity (molecular weight distribution) of polymers produced by conventional radical polymerization, and difficult to achieve a highly uniform and well-defined product. It is also often difficult to control radical polymerization processes with the degree of certainty necessary in specialized applications, such as in the preparation of end functional polymers, block copolymers, star (co)polymers, and other novel topologies.
In a controlled radical polymerization process radicals are generated reversibly, and irreversible chain transfer and chain termination are absent. There are four major controlled radical polymerization methodologies: atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), nitroxide-mediated polymerization (NMP) and iniferters, each method having advantages and disadvantages.
RAFT processes are among the most versatile controlled radical polymerization techniques, as reported by M. Freemantle, Chemical and Engineering News, Sep. 9, 2002, pp. 36-40. In a RAFT process a propagating polymer radical (Pm.) adds to a chain transfer agent (Pn-X) to generate a new radical intermediate (Pm-X.-Pn). This intermediate radical fragments either to a new propagating radical (Pn.) and a new dormant species (Pm-X), or back to (Pm.) and (Pn-X). The RAFT chain transfer agent establishes a dynamic addition-fragmentation equilibrium by transferring activity between the propagating radicals and the dormant species. The polymerization may be reactivated by addition of more thermal- or photoinitiator and monomer.
There is a need for a radical polymerization process which provides (co)polymers having a predictable molecular weight and a narrow molecular weight distribution (low “polydispersity”). A further need is strongly felt for a radical polymerization process which is sufficiently flexible to provide a wide variety of products, but which can be controlled to the degree necessary to provide highly uniform products with a controlled structure (i.e., controllable topology, composition, etc.). There is further need for a controlled radical polymerization process which provides telechelic (co)polymers capable of entering into further polymerization or functionalization through reactive end-groups, particularly electrophilic end groups.